P0400 Code: Exhaust Gas Recirculation (EGR) Flow Malfunction
🟠 Moderate severity · drivable
Drivable, but the EGR system isn't working correctly, which can cause pinging and failed emissions tests.
What P0400 means
The exhaust gas recirculation (EGR) system has a flow problem. P0400 is the general EGR fault code, less specific than the flow-direction codes P0401 (insufficient) and P0402 (excessive).
Symptoms
- Check engine light is on
- Possible pinging under load
- Rough idle
- Failed emissions test
Common causes
- most common: Carbon-clogged EGR valve or passages
- common: Faulty EGR valve
- less common: Cracked or disconnected vacuum line
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Remove and clean carbon from the EGR valve and passages | Yes | $20–100 |
| Replace the EGR valve | Yes | $100–400 |
| Repair a cracked or disconnected vacuum line | Yes | $5–30 |
Frequently asked questions
Can I drive with a P0400 code?
Yes, but the EGR system isn't working, which can cause pinging. Fix it before your next emissions test.
What is the most common cause of P0400?
Carbon buildup clogging the EGR valve and passages.
How much does it cost to fix a P0400?
Cleaning is $20–$100 DIY; a new EGR valve runs $100–$400.
Related codes
P0401
P0401 Code: Exhaust Gas Recirculation (EGR) Flow Insufficient Detected
P0402P0402 Code: Exhaust Gas Recirculation (EGR) Flow Excessive Detected
P0404P0404 Code: Exhaust Gas Recirculation (EGR) Circuit Range/Performance
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.